Biochar Wok Stove

Alacrates's picture

I try to make little projects every few months that relate to Green Wizardy subjects. I live in an apartment, so it's difficult at the moment to get very involved the the food growing aspects of the resilience toolkit (though not impossible, I could find a community garden of some sort) and I don't have a house that I can tinker with or make improvements on.

So my idea is to take on small projects that at least get me working with various principles involved in Green Wizardry: heat, food preservation, alternative cooking methods, recycling materials, space heating, etc. and to try to consider these things from a systems points of view, thinking about their inputs and impacts, and how they inter-connect with other things. My idea is to plant as many seeds in my head as I can, and maybe they'll be a grow into something larger when the circumstances call for it.

Anyways, the project that I'm working on now is a 'biochar wok stove', basically a way to sit a wok pan over top of a burner that is producing biochar as end product (instead of wood ash.) I plan to write up a blog post about it, related to some ideas I had about cooking on woks, etc. For now, I posted a short video of the project to youtube:

For anyone who is not familiar with biochar, it is pretty much similar to charcoal, it is created by heating organic matter (very often wood) in a low oxygen environment, basically baking it so that volatile gases are emitted from the wood, leaving behind char, that is largely composed of carbon. That low-oxygen heating process is called pyrolysis.

If biochar is any different than charcoal, it is because it is often made at hotter temperatures than the traditional lower-temperature smoldering processes for used for making charcoal, and because it is often made to be added into soils, while charcoal is usually made to burn. (It also differentiates it from charcoal "briquettes" which have a lot of additives in them to make them burn better, etc.)

"Labile" carbon is the kind of carbon that is easily broken down by microbes (e.g. corn stalks, fallen trees, dead animals) and returns to the atmosphere mainly as carbon dioxide, to be pulled down to the land again eventually by plant phytosynthesis in the carbon cycle. Biochar, however, is "recalcitrant" carbon, which is very resistant to decay, they say for most cases, thousands of years, if not more.

Biochar is a very porous material, with many cavities within it, which materials adhere to very easily (which is why Brita filters along with many water treatment systems and beverage companies use charcoal to filter water). It is argued that biochar, when it is added, crushed, into a compost pile, or inoculated with compost tea, or urine, or mixed into humanure systems, can provide habitat for beneficial soil microbes and be a storehouse for nutrients for plants to draw from.

Biochar also holds onto a lot of water, so some think it could be a method to improve drought resistance from soils. Ironically, it can apparently help with drainage when it is incorporated into heavy clay soils.

There is some controversy about how much of a miracle cure biochar is for degraded soils. I've often heard that it is pretty helpful in degraded tropical soils, but less so in the temperate regions of North America. But I think a lot of studies haven't taken into account the types of nutrients and microorganisms that are being soaked into the char before it is added to soils, I'm thinking that could make a big difference, but I'm no expert in gardening and agriculture so I really couldn't say

One thing that is getting me interested is seeing applications for biochar in a lot of building products, allowing us to sequester carbon and to turn organic waste materials into useful products, so that new crops can draw down some of the carbon we have put in our atmosphere by burning fossil fuels, among other things.

Anyways, I'm thinking if we can use the heat created through making biochar, so much the better, so that's the spirit behind the biochar wok stove. It's actually not a bad choice for an outdoor cook stove, they are pretty much smokeless, they create a lot less soot than other kinds. And they give a slow steady long lasting heat, it is basically cooking over wood gas, a similar sort of flame to natural gas.

Anyways, let me know what you think! Thx.

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ClareBroommaker's picture

I like your thinking!

Is making biochar a smokey process? It does make sense to use that heat coming off of the burn, but if it is terribly smokey, seems like whatever cooking you'd do might need to be completely sealed off from the smoke. And is making biochar long and slow? Then cook foods that need a long, slow, cook? Or is biochar made quickly or with particularly hot fire? Wok cooking seems like fast cooking over intense heat.

Do you suppose that in parts of the world where charcoal is the common cooking fuel, people also cook over the production fire? I'm thinking it might normally take some very large tripods to suspend a pot / wok over a big pile of wood or burn barrel. Is biochar usually made in smaller batches, over which you might set a family size pot or wok?

Incidentally, my son was saying something to me about having altered one of his gas stove rings so that he could cook over "a cone of fire." He claimed it was more efficient. I thought he meant fuel efficient, but maybe he meant time efficient. At the time I just laughed at the mental picture I had and did not ask more about efficiency. On the other hand, I thought typical wok cooking is supposed to be fuel efficient even though it uses a high blast of heat for a short time. (The cone of fire?)

ClareBroommaker's picture

Whoops. Missed your last paragraph, in which my questions are answered. (blush)

Blueberry's picture

Having lots of smoke while making charcoal if you live in the city is not a good thing. Your smoke is wood gas. If you have some type of after burner on you equipment no smoke. Several months ago did a topic in the seventh circle on making charcoal. Living in the country smoke is no problem. My set up makes app 3-5 gal buckets per burn. The winter of 2018-2019 made enough to apply nearly 60 gals of charcoal to the garden. When making charcoal for the garden will use what ever kind of wood is at hand. Cooking charcoal will use oak or pecan. The pecan is a great wood just not easy to source, mostly branches from storm damage.

Alacrates's picture

There is no smoke at all from these burners. You can't see or smell any smoke coming from the burner.

It works just like in the video for the entire process, and when the wood is all turned to char, the flame just goes out (and at that point you have to either quench it with water or completely seal it from oxygen.

This burner uses a "TLUD" design (Top Lit Up Draft), basically made so that a small amount of combustion air comes in at the bottom of the unit, is heated as it rises between an outer and inner chamber, and then combines to ignite with the wood gas and then drawn out the chimney.

These units do burn the wood gas that is being produced by the pyrolysis, that flame is what is heating the rest of the wood to release it's wood gas (and what is creating the heat for the wok pan in this case.)

I don't know a lot about charcoal, but I'm thinking this might be a difference between traditional charcoal making and what they are calling "biochar". The aim for all these is to burn at around 500 - 800F, and to produce no smoke. I believe this is because, A) they are aiming for carbon sequestration, so they are trying to keep as much carbon out of the released gases as possible, and B) because the char it produces has more pore spaces and less tars within them.

I make small units because I don't to have to keep an eye on a flame for more than 2 hours or so, and I'm really making these as an experiment, but I guess they do sequester carbon whenever they are used, I just want to do a bit more research on biochar before I start offering it to be added to the composters that I know. There are a lot of larger, farm-scale biochar set ups that people are using, the most common ones I've seen online are the 55-gallon retort drums, and the very cool kon-tiki kilns.

Blueberry's picture

So when is lunch? Like your set up was unable to get u tube to work this morning during the storms. Would you be willing to do a tutorial on the making of your cooking stove. Have sent a link to people in different parts of the world.

Alacrates's picture

When you come by Winnipeg, I'll guarantee the lunch, hehe.

I did take pictures, and was thinking of making something for But really it's just an alteration on the TLUD, which there are many online.

My favorite overall tutorial, especially for people in the country or on farms is this one, it's a bit long, but it's easy to skip forward to their burn barrel set up if needed:

I also really like the kon-tiki kilns, that use an open fire in a cone shape kiln, that use the natural air currents this shape sets up to keep the charcoal from oxidizing - strange too that they kind of look like giant woks!

Alacrates's picture

Hehe, yes, it makes no smoke at all. You can see some hot air rising from the chimney, and you can't see or smell any smoke, none of that campfire smoke smell on your clothes after either.

I did have to tinker with the air holes and the inner chamber to get it so that it would burn smokelessly at first it would burn well for 30 min. or so, and then start to smolder, releasing a huge stream of smoke, me and my girlfriend went back into the house smelling very smokey that day!

You're right though, at the moment that burner is not giving off enough heat to really properly stir-fry, it is more like a simmering temperature. I have make other biochar makers, and the flame is always 500 - 600F, so I thought I was good, but I didn't account for how much that smaller vent would whisk the heat up and out of the unit.

Luckily I have enough materials to make a second wok unit, so on that one I'm not having a Y-fitting, the heat will be closer to the pan and directly underneath it. My girlfriend suggested we just modify the first one (in the video) so that it could it could simmer over a longer period (maybe some braised chicken thighs or pork shoulder) while we do shorter cooks on the new one.

You're right too that wok cooking is often based around short bursts of high heat. I was going to write about this more in the blog post I was planning, but I got the idea from these from reading a few books on wok cooking, and they had these descriptions of street vendors in Asia, who take a large barrel of charcoal and cook their food over it for the day, often times while their partner is beside them cutting vegetables and taking payments.

I've done wok cooking in a restaurant that had a really nice double wok set up (it had an adjustable knee pedal that you could adjust the burners underneath it to the heat you wanted, leaving both hands free to work) and I really liked that way of cooking, in my peak oil doomsday fantasies I thought maybe I would try the charcoal barrel down on main street if times ever got tough, haha!

When I learned about biochar, the idea came to me, could that long burning barrel style be done while making biochar, instead of using up charcoal, sequestering carbon and producing a soil ammendment while cooking meals for people? And when I have an idea like that I can't let it go until it's done.

I've thought maybe I could set one of these up at a farmers market, and make stir-fries while promoting the idea of biochar, but I'm not sure where I'd find the time for that, and if anyone would let me do that anyway!

Alacrates's picture

Oh yes, I forgot to mention too, there are a lot of projects to provide these pyrolytic stoves to places where people cook over wood fires, often in semi-enclosed spaces, where soot inhalation is a big problem, especially for children.

These biochar stoves are supposed to be a lot more efficient in fuel use, and give off very little soot, and if people can find uses for the charcoal (as biochar to add to the soil, or as filler for things like plaster, mortar, etc.) then all the better.

I never fully understand how people cook indoors on open flames, these biochar stoves, from what I understand, can give off a lot of carbon monoxide, there are always warnings not to use them indoor (not that I would be tempted to do that!) I'm guessing that these homes in the warmer places are pretty open to the air, and maybe have some sort of chimney in the roof to let out some gases?

Blueberry's picture

Indoors in many parts of the world is very open. If not using wood in a open fire, would be using a kero cook stove. the folks using kero are considered well off!!! I own one of these and would never use indoors, my house is to tight!

David Trammel's picture

Very nice!

When I move into my retirement location, Sister wants a outdoor patio. I'll have to build one of these for barbeques and use the biochar for the backyard garden.

Alacrates's picture

They're actually quite easy to make, I'll try to write up a tutorial for this one.

I started with a few small biochar stoves, and got a basic idea of how many holes to drill, and where to locate them. I still haven't seen anything online that gives exact information on how many air hole that one should have. So I've had to tinker a bit until the air flow worked well.

The simplest one, that in some ways is my favorite for a little patio fire because you can see the flames easily, and they give off nice light patterns through the triangles in the lid, is the "Toucan" stove, that Hugh McLaughlin shows how to make in a pretty good youtube video. All you need is an empty paint can, and two other tin cans (which is why it is called a "two can" stove!) It gives off about 1.5 hours of fire, and by the end you have a little mass of biochar, which one can put to use, and which represents a certain amount of carbon that is not going back up into the atmosphere.

David Trammel's picture

I understand the benefits of biochar as a soil additive but what are the advantages of cooking with charcoal as opposed to wood?

Alacrates's picture

I probably should have been more clear on this (I am trying to write up a few blog posts on some projects I've tried with biochar) but this stove uses wood to create the flames, and the end result is biochar. The idea is that this biochar is not to burned as charcoal (in which case it combines with oxygen and the carbon is released into back into the atmosphere) but the biochar is for adding into compost, etc., and buried in the ground.

There are advantages to cooking with charcoal, I'm not an expert in in that at all, I've really never cooked over real wood charcoal, just the briquettes which I think have a lot of additives combined into them. I know that charcoal was important throughout history as it was a way to produce high heat fires that were necessary for metallurgy. Also in damp environments, wood can rot, charcoal is a good way to store larger amounts of cooking fuel. I think it burns with less smoke/soot as well.

Or another idea I'm excited about is using biochar powder in building materials - concrete, asphalt, mortar, plaster, paint, and many others. The carbon has a lot of useful properties, it cuts down on mined products like construction sand, and we can sequester carbon with everything we build.

So with these biochar producing stoves, there has to be some way to heat up the wood in a low oxygen environment (aka pyrolysis) so that it begins to give off volatile gases, which are flammable. This way the oxygen doesn't combine with the carbon in the wood, to form carbon dioxide, a greenhouse gas. In these types of stove, basically there is an inner chamber that I put wood pellets in, and the outer chamber, where some air is allowed in, it heats up along the walls of the inner chamber, and combusts with the volatile gases given off by the wood to burn above the wood pellets - the heat from those flames keep baking the pellets, and in the end you are left with biochar.

It took me awhile to get a handle on the processes involved in biochar, but I encourage anyone who is interested to make a simple Toucan burner, it demonstrates what is involved so simply, it really gave me a feel for what pyrolysis is, so I could work on larger units.

Blueberry's picture

If you like a good steak, chops, chicken charcoal is the must have fuel. When cooking over a open fire you need a bed of coals before cooking food. Easy to control the heat most folks when learning to cook on a open fire have to much fire. One can use charcoal and a pot as a slow cooker Nothing like home made soup cooked over charcoal for several hours. Baked potatoes or onions cooked directly in a bed of coals.

Alacrates's picture

I've never cooked with actual wood charcoal. Just wondering if it is fairly smokeless? I'm thinking it must be, because for people who have charcoal stoves, I think you add a pack of wood chips (mesquite, hickory, etc.) if you want to add smoke flavor to the meat?

Also curious, how do you light and sustain the fire? Do you just start a few of the charcoal pieces to combust with a match, and then the whole mass of charcoal will start heating up, like with the briquettes?

David Trammel's picture

I know with the small round brickettes that you burn incense or herbals, like sage or sweet grass, you put a match to one side of the brickette. It catches fire but without any flame. Its interesting to watch as the ignition works its way across the brickette. Then its hot. Perhaps there is an added chemical to make it readily ignite. Someone with experience with real charcoal could say.

Blueberry's picture

Biochar is real charcoal. So how to light could use charcoal lighert NOT!!!! Charcoal lighter is just mineral spirits. I just build a small fire with sticks and place the charcoal on top. In about 10 minutes time to start cooking. The stuff in a bag from the store is compress to take up less space per pound of weight. So stuff is add as a binding agent. Each brand is different, so MSDS is your friend.

David Trammel's picture

I was wondering on the energy comparisons between wood and charcoal. And perhaps ease of storage.

Alacrates's picture

Continuing from my last response, of course these biochar stoves do not use charcoal for heat, they use wood (or other dried organic matter) to produce heat, not really through combustion, but through pyrolysis, which makes biochar.

(There are arguments as to whether people should even use the word biochar instead of just calling it charcoal. I'm sympathetic to that, I kind of prefer tradition to innovation (!!) but I kind of support the word biochar, in particular because it introduces the idea of carbon sequestration. In addition to that, many claim that biochar is usually made at hotter temperatures, it is intended for adding to soil/building materials instead of for burning, and it differentiates it from charcoal briquettes, which have a lot of additives that make it unsuitable for adding to compost/soil)

As for energy comparisons, I'm just going by instinct, but I have a feeling that pyrolysis could be efficient compared to combustion. As far as I know, combustion burns faster and hotter, around 1000F in a roaring campfire, where as the flames in biochar stoves are usually around 500 - 800F. So it is a slower heat, but adequate for cooking food, and I'm thinking it is steadier and more long lasting, so for some cooking processes, it could be better suited than a wood fire. It also gives off much less soot, which is a major problem with many stoves throughout the world, many children get asthma from inhaling soot from combustion cook stoves.

On top of that, with a biochar stove, you are left with biochar. If that can be added to compost and help with the health of garden soils, that is more benefit drawn from that pyrolyzed fuel. (There are arguments as to how much biochar helps soil fertility, and in what kind of soils.) Or if you can use it in building material, that is another efficiency - maybe a local company could buy up the char people are making? And if we are going to put money as a society into carbon sequestration, this is a way a person can get utility from the fuel while at the same time drawing down carbon.

As for the heat of biochar wok stove, I really did not need that vent coming out at an angle, I lost too much heat from that, not enough was directed to the pan. But my girlfriend thought we should leave that stove to be a slow cooking biochar stove (for braising meats, making bean dishes, etc.) and then build another one for fast, intense cooking. I'm hoping to finish that second one this weekend, I'll post it here if I do, it will be a better wok type stove!

Alacrates's picture

Over the weekend, I tried a second design of the biochar wok stove design, and made a little youtube video of the results. I also had to use some 2x4 lumber cut-offs as fuel instead of the wood pellets that I normally use, it turned out well. I made some stir-fried vegetables, and some braised chicken thighs in a thai curry sauce.

I think the two models work well together. The first, that had a vent on a Y-fitting, didn't really get hot enough for a proper stir-fry, but it has a nice moderate temperature for simmering a braised meat in sauce. The second wok stove had really intense heat under the wok, great for searing meats and a quick stir-fry of vegetables. If I light both of these stoves at the same time, I can use the two different heats to make a good meal.